Some display systems, for example, liquid crystal displays (LCDs), are illuminated from behind. Such displays find widespread application in many devices, e.g., laptop computers, hand-held calculators, digital watches, televisions, and the like. Some back-lit displays include a light source that is located to the side of the display, with a light guide positioned to guide the light from the light source to the back of the display panel. Other backlit displays, e.g., some LCD monitors and LCD televisions (LCD-TVs), are directly illuminated from behind using a number of light sources positioned behind the display panel. This direct lit backlight arrangement is increasingly common with larger displays because the light power requirements needed to achieve a certain level of display brightness increase with the square of the display size, whereas the available real estate for locating light sources along the side of the display only increases linearly with display size. In addition, some display applications, such as LCD-TVs, require that the display be bright enough to be viewed from a greater distance than is required for other display applications. Further, viewing angle requirements for LCD-TVs are generally different from those for LCD monitors and hand-held devices.
LCD monitors and LCD-TVs are commonly illuminated from behind by a number of cold cathode fluorescent lamps (CCFLs). These light sources are linear and stretch across the full width of the display, with the result that the back of the display is illuminated by a series of bright stripes separated by darker regions. Such an illumination profile is not desirable; therefore, a diffuser plate is typically used at the back of the LCD device to smooth the illumination profile.
Currently, LCD-TV diffuser plates commonly employ a polymeric matrix of polymethyl methacrylate (PMMA) with a variety of dispersed phases that include glass, polystyrene beads, and CaCO3 particles. These plates often deform or warp after exposure to the elevated temperatures caused by the lamps. In addition, these diffuser plates require customized extrusion compounding to distribute the diffusing particles uniformly throughout the polymer matrix, which further increases costs. Some diffuser plates are provided with a diffusion characteristic that varies spatially across its width in an attempt to make the illumination profile at the back of the LCD panel more uniform. Such non-uniform diffusers are sometimes referred to as printed pattern diffusers. These non-uniform diffusers are expensive to manufacture because the diffusing pattern must be registered to the illumination source at the time of assembly.
Recently, liquid crystal display television sets (LCD TVs) have been introduced that use a direct-lit backlight powered not by CCFLs but by an array, e.g., of red/green/blue LEDs. An example is the Sony™ Qualia 005 LED Flat-Screen TV. The 40 inch model uses a direct-lit backlight containing five horizontal rows of side-emitting Luxeon™ LEDs, each row containing 65 such LEDs arranged in a GRBRG repeating pattern, and the rows being spaced 3.25 inches apart. This backlight is about 42 mm deep, measured from the front of a diffuse white back reflector to the back of a (about 2 mm thick) front diffuser, between which is positioned a flat transparent plate having an array of 325 diffuse white reflective spots. Each of these spots, which transmit some light, is aligned with one of the LEDs to prevent most of the on-axis light emitted by the LED from striking the front diffuser. The back reflector is flat with angled sidewalls.
An important aspect of some backlights is that the light illuminating the display panel should be uniformly bright. Illuminance uniformity is particularly a problem when the light sources used are point sources, e.g., LEDs. In such cases, the backlight is required to spread the light across the display panel so that the displayed image lacks dark areas. In addition, in some applications, the display panel is illuminated with light from a number of different LEDs that produce light of different colors. Because the human eye more easily discerns variations in color than in brightness, it can be difficult to effectively mix light sources that produce different colors to produce white illumination light. It is important in these situations that the light from the different LEDs be mixed so that the color, as well as the brightness, are uniform across the displayed image.